WERA is a
HF radar
using electromagnetic waves between 6 and 30 MHz
(50 m to 10 m wave length) to measure surface current velocities,
ocean wave height (spectra) and wind.
The transmitted vertical polarized electromagnetic wave
travels along the sea surface beyond the horizon and is backscattered
by ocean waves of half the electromagnetic wavelength (Bragg Scattering).
Typical working ranges are 15...30 km for ocean waves and
40...50 km for surface currents
at 27.65 MHz working frequency, 35 PSU salinity and medium sea state.
Here is a table on working ranges
and further remarks on other radar frequencies.

As one WERA radar measures radial components,
two or more radars are required
to measure the full 2-dimensional values.
The distance of the two WERA radars should be around 15 km
at 27.65 MHz, if sea state measurements are required.
This is to ensure a sufficient signal-to-noise ratio for the wave algorithm.
If only surface currents are to be measured, increasing the distance to
around 35 km reduces the measurement errors due to geometry.

To decide on the measurement area and the radar sites, a nautical map is
required to know about ship traffic, light houses, and water depth / bottom
topography structrue. A topographical map is needed to plan site logistics
like access by roads as well as the height of land and islands.

The figure shows an example of a WERA installation.
If a linear receive antenna array is used, this should be installed parallel
to the coast to minimize the influence of the signal path over land on the
receive antenna beam pattern.
Receice and transmit antennas should be set-up on a straight line to ensure
the null in the transmit antenna pattern
to point to the receive antenna.
The required minimal distance between transmit and receive antennas is
100 m to ensure sufficient isolation (>70 dB) on the direct path
(WERA uses FMCW, i.e. transmitter and receiver are operated simultaneously).

The area (angle) covered by the HF radar is ±60° perpendicular to the
receive antenna array, if a linear array is used for receiving.
In case of the 4-antenna square array (surface currents only, no ocean wave
measurements possible), the covered area is given by the
transmit antenna pattern.

To maximize the working range, the antennas should either be installed as
close to the water as possible, or on top of a cliff.

Depending on the working frequency used by the radar, antenna height
and spacing varies. For a working frequency of 27.65 MHz, antenna spacing
is 5.42 m (half the electromagnetic wavelength). The antenna height
of a full length quater-wave groundplane is 2.7 m. Below 25 MHz,
the antenna must be electrically shortened using a coil or a wound wire
to ensure enough mechanical stability to withstand stroms.

The transmit antenna consists of two rows of 2-element linear array antennas
with 0.5 Lambda spacing. The two rows are 0.15 Lambda apart. To form the
antenna pattern, the cables from the
power splitter to the antennas have to be cut to a specific length. The cables
A and D (to back row) have to be 0.35 Lambda shorter than the cables B and C
(to front row). The propagation speed factor for RG213/U is V = 0.66 and
has been
taken into account for the calculations of the values given in the table below.

Frequency

0.5 Lambda

0.15 Lambda

Length difference A,D to B,C

Cable length A,D

Cable length B,C

29.85 MHz

5.03 m

1.51 m

2.32 m

6.63 m

8.958 m

27.65 MHz

5.42 m

1.63 m

2.51 m

7.16 m

9.678 m

16.045 MHz

9.35 m

2.80 m

4.32 m

12.34 m

16.66 m

12.50 MHz

12.00 m

3.60 m

5.54 m

15.84 m

21.38 m

8.00 MHz

18.75 m

5.63 m

8.66 m

24.75 m

33.41 m

Note, that the length of cables A and D can be selected to some practical
value (to mechanically install the power amplifier at a convenient pace) and
cables B and C have to be 0.35 Lambda * V longer.

The null produced in the antenna pattern
should point towards the receive
antennas to reduce the energy transmitted on the direct path from the
transmit to the receive antenna. The transmit power amplifier, which also
houses the power splitter, is located at the transmit antenna.
A 200 m long RG 213/U cable is required for
the transmit radio signal, a power cable (115/230 V, 3*2.5 mm²)
is required for operation of the power amplifier.

If the transmit antenna can not be installed to direct the null in the antenna
pattern towards the receive antenna, a second null can be generated 42 degrees
to the back side of the transmit antenna by keeping the mechanical
distances of 0.15 * 0.5 Lambda, but increasing the electrical cable
length difference from 0.35 Lambda to 0.40 Lambda. Here is the
resulting antenna pattern.

If sea state is to be measured, e.g. significant waveheight and wave
directional spectra, a 16-element linear array is required.
A 12-element linear array can also be used, but gives some coarser azimuthal
resolution. The total
length of 16-element array is 15 * 4.88 m = 73.24 m at 27.65 MHz (0.45 Lambda spacing).
The antenna spacing should be configured for the highest frequency used by the system.
The antenna spacing and variances in height of the mounting points should be
within 1 % accuracy.
We often use wooden sticks of ~5 cm * ~8 cm to fix the
antennas to, so fine adjustments can be done during installation.
Each antenna is connected to a separate receiver channel, so 16 cables
RG 213/U, each 200 m long, adjusted to ±20 cm difference
in length, are required.

Frequency

Spacing

16-element total length

12-element total length

29.85 MHz

4.52 m

67.84 m

49.75 m

27.65 MHz

4.88 m

73.24 m

53.71 m

16.046 MHz

8.41 m

126.20 m

92.55 m

12.50 MHz

10.80 m

162.0 m

118.8 m

8.00 MHz

16.88 m

253.13 m

185.63 m

The direction of the array is required to correctly map the measurements
to geographical coordinates.
This is done by passing "True North"
to the processing software.

If the linear array is operated in beam forming mode, these are the
expected antenna patterns (beams) for 16 antennas when steering the beam to
0 degrees
(perpendicular to the array) or
45 degrees. Note, that due to the
reduced active aperture length, the beam gets wider when steered to the side.

The following requirements have to be met to ensure proper operation of WERA:

The 16(12)-element linear receive antenna array must be installed parallel
to the coast, e.g. all distances from the antenna to the sea must be the same.
This ensures that all antennas get the same receive energy of the backscattered
signal. Variances in distance from the antenna to the sea may be compensated
by changing the antenna height above sealevel, allowing to set up the array
at some slight angle to the coast line.

Install the antennas as near to the water and as high as possible. This
increases the coupling of the antennas to the sea surface, resulting in
stronger signals backscattered from the sea.

There must not be any metal fences, power cables or other electrically
conductive material within 5 electromagnetic wavelengths distance
around the antennas. Also, avoid electromagnetic noise in the HF band generated
i.e. by motors or high-voltage power lines.

The WERA radar itself should be installed in a metal container to keep
local computer noise off the receive antennas.

Look for a free operating frequency; avoid interference with local
wavebouys, which may be operated within the HF radar bandwidth.

Wavebouys may receive and retransmit the HF radar signal if they are
located too near to the transmit antenna. Give at least ~1 km distance
between transmit antenna and wavebouy.

The radar and antenna installation should be closed to public.

The save distance to the transmit antenna for people using a pacemaker
is 3 m at 30 MHz, less than 3 m at lower frequencies.

The antenna systems should be installed high enough to be protected
against storm surges, waves, spray and foam.

The container should be air conditioned to keep the temperature at
18...23 °C. This may take up to 5 kW and depends on the
local conditions (cooling or heating).

A data link between the WERA radars or the WERA radars and an operation
center is required to on-line calculate 2-dimensional surface current
fields. A ISDN telephone link would be sufficient, if no raw data
need to be transferred. The size of a file containing radial components
of the surface current at 60*60 grid points is 101,888 bytes. Doppler
spectra at 60*60 grid points, which are required for the ocean wave
algorithm, take 7,373,312 bytes. To transfer this file within 5 minutes,
a transfer rate of 25 kbytes/s is needed, which is faster than ISDN
(7.5 or 15 kbytes/s).